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Federal Court Considers the Admissibility of Expert Reply Evidence

January 13, 2021


In Merck Sharp & Dohme Corp v Wyeth LLC (2020 FC 1087) the Federal Court granted leave for the plaintiff to serve and file two expert reply reports and one expert rebuttal report. In doing so, the Court explained the circumstances in which expert evidence is being introduced as proper reply rather than an attempt to split a case.


Merck Sharp & Dohme Corp. and Merck Canada Inc. (“Merck”) brought an impeachment action against Wyeth LLC (“Wyeth”) in which they sought an order invalidating a composition patent and two formulation patents relating to a pneumococcal conjugate vaccine with 13 serotypes. Both of the parties filed three expert reports in support of their respective positions.

One week before trial, Merck brought a motion seeking leave to serve and file three additional expert reports relating to the formulation patents and the composition patents and one rebuttal report relating to Wyeth’s prosecution history with the Canadian Intellectual Property Office.

The Federal Court’s Decision

In determining whether Merck should be granted leave to serve and file the additional expert reports, the Federal Court summarized the principles relating to the ability to introduce reply evidence and the principles governing the admissibility of expert evidence.

The Court in Merck recognized the general prohibition against case splitting, which precludes plaintiffs from raising evidence in reply that ought to have been raised in their case in chief. Notwithstanding this general prohibition, the Court explained that judges retain discretion to admit such evidence where they are satisfied that it constitutes proper reply evidence. Determining whether evidence is proper reply requires judges to consider whether the evidence is responsive to the other party’s evidence and whether its relevance could have been anticipated at an earlier stage in the proceedings.

The Rebuttal Report

First, the Court granted leave to Merck to file the rebuttal report.

The rebuttal report provided expert evidence related to Wyeth’s communications with the Canadian Intellectual Property Office about the process of creating a conjugate vaccine. According to Merck, the evidence presented in Wyeth’s expert reports contradicted these communications. Ultimately, the Court was satisfied that leave should be granted as the rebuttal report would benefit the trier of fact and related to issues that Merck could not have anticipated before Wyeth filed its expert reports.

The Reply Reports

The Court then granted Merck leave to file each of the reply reports.

Each of these reports discussed various scientific issues raised in Wyeth’s expert reports and the parties disagreed as to whether Merck could have reasonably anticipated these issues at an earlier stage in the proceedings. In determining that Merck could not have reasonably anticipated the issues, the Court explained that “a plaintiff is not expected to try to anticipate every argument of the defendant; to do so would hinder the efficiency of a well-circumscribed litigation and would force a plaintiff to file unnecessary and burdensome evidence.”

In granting leave to Merck, the Court rejected Wyeth’s claim that one report should be excluded because its evidence was duplicative of another report already in evidence. When rejecting this claim, the Court explained that reply reports should not be refused on the basis that they contain evidence that complements other expert evidence as this would effectively preclude different experts with different areas of expertise from ever giving evidence on the same subject for the same party.


Authors: Jaclyn Tilak and Rachel Oster


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